One example of a wireless communications network using contention-based transmission resources of the same frequency is the standardized IEEE 802.11 Wireless LAN, WLAN. Here, a Basic Serving Set, BSS, is regarded the basic building block of the wireless communications network. The BSS comprise an Access Point, AP, and a number of stations, STAs, located within a certain coverage area or cell being served by the AP. Within a BSS, the transmission between the AP and the STAs is typically performed in a distributed manner. This means that before a transmission, a STA first performs a Clear Channel Assessment, CCA, by sensing the transmission medium for a specific period of time. If the transmission medium is deemed idle, then access is assigned to this STA for transmission; otherwise, the STA typically has to wait a random back-off period and then again check whether the transmission medium is idle and thus available to the STA. The random back-off period provides a collision avoidance mechanism for multiple STAs that wish to transmit in the same BSS. In this case, the above contention-based channel access is commonly referred to as a distributed coordination function, DCF, in the IEEE 802.11 WLAN standard.
In order to avoid interference in wireless communications networks using contention-based transmission resources of the same frequency, different frequencies, or channels, should be assigned to neighbouring or nearby BSSs. However, in dense deployment scenarios, it is likely that frequencies or channels will be reused even for neighbouring or nearby BSSs. In this case, resulting co-channel interference between the BSSs is expected to compromise the performance or Quality-of-Service, QoS, offered to the STAs by the BSSs. In particular, STAs that are located within an overlapping coverage area of the BSSs may, due to relatively strong interference, be more severely affected. In the IEEE 802.11 WLAN standard, this is commonly referred to as having Overlapping Basic Service Sets, OBSSs.
One problem created by co-channel interference is that when the interference from one BSS is not adequately strong to influence the channel access in another BSS, simultaneous transmission in the overlapping coverage areas will lead to a lower signal-to-noise-plus-interference, SINR, ratio in both BSSs, and therefore affect the performance and QoS in both BSSs. Another problem created by co-channel interference is that due to the contention-based channel access in the BSSs, the interference from a first BSS could be detected as the frequency or channel being busy by a second BSS, whereby the second BSS may defer its transmissions by mistake. In extreme cases, continuous co-channel interference may block the transmission at several BSSs. Hence, this type of co-channel interference in such dense deployed wireless communications networks will lead to significant losses in performance and QoS and thus should preferably be mitigated.
To handle this type of co-channel interference, several power-control related ideas have been proposed, in particular those with coordinated power control. One example is “Advanced power control techniques for interference mitigation in dense 802.11 networks”, O. Oteri et al, 16th International Symposium on Wireless Personal Multimedia Communications (WPMC), 2013. Here, a coordination between AP and STAs in an OBSS is described which allows for scheduling high-power transmissions in different time slots. This type of solution may be limited in dense deployment scenarios since the overlapping high-power transmission become frequent as the number of co-channel STAs increases. Other types of solutions based on coordinated power control, on the other hand, may sacrifice the SINR towards the STAs.
In the IEEE 802.11ac WLAN standard, beamforming and determining how to radiate energy in a desired direction is described. This is performed using a channel sounding procedure based on a Null Data Packet, NDP. The channel sounding procedure based on NDP is shown in FIG. 1.
Initially, a beamforming AP will transmit a NDP Announcement Frame, AF, in a first time slot. This is performed in order to gain control of the channel, i.e. the contention-based transmission resources. The targeted STA will respond to the AF, while other STAs will defer channel access in order not to interfere. The format of the AF according to the IEEE 802.11ac WLAN standard is shown in FIG. 2.
Then, the beamforming AP transmits a NDP frame in a subsequent second time slot. The format of the NDP frame according to the IEEE 802.11ac WLAN standard is shown in FIG. 3. Here, it may be seen that the NDP frame is equivalent to a regular frame in the wireless communications network 100, but with no data payload part. The NDP frame mainly comprises training signals through which the channel towards the beamforming AP may be well estimated by the targeted STA.
The targeted STA will then estimate the channel towards the beamforming AP through the training part of the received NDP frame and transmit the result of the channel estimation in a subsequent third time slot. The result of the channel estimation may be carried in a so-called Feedback Frame as shown in FIG. 1. Upon receiving the result of the channel estimation, the beamforming AP may determine the beamforming directions.
US2004/0056204 A1 describes method for interference alignment in an OBSS of a WLAN which uses the above mentioned beamforming and channel sounding procedure based on NDP in order to handle interference in the OBSS. However, the method does not take all interference that may occur in the OBSS into account when performing the interference alignment. Hence, the achievable interference mitigation is limited.